The Mets’ first-base shuffle might continue longer than the club originally anticipated.

Lucas Duda’s stiff lower back hasn’t progressed as quickly as the Mets would have hoped, leaving it unclear if he will be in position to return from the disabled list Sunday when he is eligible.

“We don’t have any day in mind, be he hasn’t hit — he is still tracking [pitches],” manager Terry Collins said before the Mets’ 3-1 victory over the Phillies on Monday. “I don’t know how close he is. I know he gets his therapy inside and goes out and he’s doing what he should be doing.

“He’s played catch, so that was a good sign. He played catch [Monday] and tracked another 250 balls off the machines so he is doing stuff, he just hasn’t swung the bat.”

Duda was placed on the DL on Aug. 22 with stiffness in his back and later received a favorable prognosis from Dr. Robert Watkins, who examined the first baseman in Los Angeles.

Daniel Murphy and Michael Cuddyer have handled first base in Duda’s absence. Collins also planned to use Juan Uribe at the position, but recently scrapped the idea because the veteran infielder indicated he was more comfortable playing second with Murphy in the lineup.

Steven Matz, Kevin Plawecki, Kirk Nieuwenhuis, Eric Campbell and Eric Young Jr. are slated to join the Mets on Tuesday when the rosters expand. In addition, Bobby Parnell likely will be activated from the disabled list. To make room for Young on the 40-man roster, Danny Muno was out-righted to Triple-A Las Vegas after clearing waivers.